The key problem as I see it is that the R/R in a moto is typically a shunt type, i.e. shorts the stator to ground to control voltage. So instead of diodes in the R/R frying perhaps it is the mosfet or whatever electrical device that does the shunting that gets fried because the alternator will see it as a short and drive it to failure.
That is a good point dmfdmf. You zeroed right in on the weak spot. Shunt power has the potential to be a problem though might not actually be an issue.
Referring to the SV650 charging diagram above, the electrical parts that would be stressed would be the SCRs and the Stator coils. The rectifying diodes and the Control IC would not be affected.
Suppose the running car puts out, say, 14.5v, but the SV650 regulator SCRs are programmed to shunt at 14.0v. Seeing 14.5v would trigger the SCRs to continuously short the stator during the entire time the car is connected.
Damage to the SCRs and stator would depend on 1) Shunt duration, 2) How much power the stator is producing while shunted 3) How much margin is in the SV650 R/R SCR and stator design.
-Presumably, the duration would be short. Connect the jumper cables, start the bike, remove the jumpers.
-Stator power output would be low at cranking and idle rpm. The only time the stator would generate significant power is if the SV engine were held revved.
-How much design margin is in the SV650 R/R and stator might be inferred by SV650 race bikes. Race bikes run without lights, minimal electrical load, so the SCRs would be shunting a good amount of time. Additionally, the rpm of race bikes often pushes redline, so stator power output would be near max. Some of these SV bikes do 24hr endurance racing without blowing up the stator or R/R, so it seems there is a fair amount of margin for long-duration shunting at high Stator power.
Conclusion, the SCRs and Stator are not likely to be damaged.
As for other motorcycle electrics being damaged, given the motorcycle and car have the same voltage specs, why would the motorcycle be damaged and not the car?
Just because a car battery has massive current capacity relative to a motorcycle is immaterial. As Ziff said, given the same 12v charging systems, the bike will only draw the current needed. A 20A wall outlet can power a 10A hairdryer but will not blow up a mA nightlight. For a given voltage, the load defines the current. Ohm's law.
This topic is a fun intellectual discussion but basically moot. Even if jump-starting a motorcycle from a running car has zero chance of causing damage (likely true, no damage), a car battery has sufficient power to crank a motorcycle starter without the car running. No need to start the car.